GB2128404A - Piezoresistive transducer - Google Patents

Description

1 GB 2 128 404 A 1

SPECIFICATION

Piezoresistive transducer Generally speaking, this invention relates to electromechanical transuducers for converting mechanical movements or displacements into electrical signals. More particularly, this invention relates to an improved strain sensitive element or force gauge for use in such mechanical transducers.

In electromechanical transducers of the kind to 75 which the present invention is directed, a transducing element is utilized for detecting the relative displacement of two parts and for developing a corresponding electric signal. Such relative displacements have been measured in the 80 past with various kinds of strain gauges. However, these generally have a tendency to be of considerable weight, and some are very bulky and some are not very sensitive. Those that are sensitive typically have intricate designs which 85 are very expensive.

The present invention is directed to a force type sensor or gauge which is mounted between two parts between which a force is applied, and the gauge is strained in an amount which depends upon that force. As such piezoresistive transducers have been developed over the years, it has become increasingly desirable to have extremely small sensors of high sensitivity and low bulk. However, in order to develop force gauges which are of extremely small size, difficulties arise in the handling thereof for subsequent mounting upon their substrate, once they are developed. They are difficult to handle not only because of their small size, but also because of their fragility.

One of the primary advantages of force transducers lies in the fact that the displacement between the pads at each end thereof produced by relative motion of the two parts to which the 105 pads are attached is concentrated in the 11 suspended- portion of the force gauge which can mechanically amplify the strain being sensed or measured. Furthermore, the resistance change of the element per unit displacement is greatest 110 as the length of the element is reduced. By use of both short gauge lengths and appropriate leverage, very large resistance changes may result from very small displacements. This change in resistance is determined by means of electrical 115 current flowing through the element from one pad to the other, and measuring changes in voltage or other electrical properties resulting from changes in resistance. However, when attempts are made to reduce to a smaller size such force gauges, 120 then, as mentioned above, difficulties arise in respect of the handling thereof in mounting upon their substrates, as well as other problems which ordinarily arise in handling very small objects.

With this invention, by contrast, strain sensitive elements can be provided in the form of force gauges which are derived from the substrate upon which they are subsequently supported in use.

That is, the gauges can be defined upon the substrate or marked thereon, and subsequently etched right from the material of the substrate. In one form of force gauge of the invention, the gauge is etched to allow a small support or mesa underneath, while maintaining the gauge still connected by this minute portion of the substrate to the substrate proper. In its preferred form, the invention is directed to a force gauge which is etched free of its substrate along its length but continuous with it at its ends. Thus, the gauges of the invention are crystallinally continuous with their support.

That is, force gauges of substantially smaller strain volume are produced by defining the gauge in the substrate or in material rigidly bonded to the substrate, and subsequently etching away the immediately adjacent material, leaving the gauge free in space, after the fashion of force gauges of the past, but supported against unwanted cross loads by remote portions of the substrate. Such gauges may have a volume as small as 3 x 10-10 cubic centimeters of stressed material, as opposed to present commercially available force gauges wherein the strained volume is 5 X 107 cubic centimeters. Both gauges would typically be strained to one part per thousand. The strain energy is thus a thousandfold less for the smaller gauge.

It will be appreciated, in this connection, that the volume of the gauges formulated according to the present invention can vary widely depending upon ultimate use. for example, a sturdy gauge may be 3x 10-4 times 8x 10-4 times 32 x 10-4 centimeters or 10-1 cubic centimeters. On the other hand a delicate gauge may be 0.3x 10-4 times 3x 10-4 times 12 x 1 0-4 centimeters, or 10-11 cubic centimeters. It is within the purview of this invention to obtain a gauge volume of 10- 12 cubic centimeters utilizing electron beam lithography.

in considering the conditions generally preferred for carrying out a process of the present invention for producing a force gauge, a conventional silicon crystal material is selected, and the outline of the gauge is etched on the selected crystal which forms the substrate. An etch is selected which is both anisotropic and doping-selective. Caustic, hydrazine, and pyrocatchol etchants may be selected, depending upon the results desired. They attack silicon rapidly in the [1121 direction, moderately rapidly in the [1101 direction, and very slowly in the [1111 direction. With this invention, the substrate orientation is (110) plane and [ 111] along the gauge so as to define a groove over which the gauge extends. With such orientation, a groove is produced with walls which are nearly vertical, and with floors that are nearly flat.

The same etchants which are anisotropic are dopant selective, in that they attack very slowly silicon in which a boron concentration is developed which is greater than 5x 1 019/cc. In accordance with the process of the invention, the gauge is defined and its terminals are also defined by a planar diffusion or ion implantation though 2 GB 2 128 404 A 2 an oxide mask to a boron concentration of roughly 1021/Cc. The boron makes the gauge P-type, while the susbtrate is N-type. The diffused area is electrically isolated from the substrate by a P-N junction. During the etching procedure which forms the groove, the gauge is exposed to the etchant, but is resistant to it. As will be appreciated, and explained further herein, when the groove is defined over which the gauge extends, a hinge is also defined in the substrate around which one end of the substrate moves relative to the other to develop the strain being monitored by the sensor. Also, the hinge protects the gauge against transverse loads.

As a further feature of the invention, two substrate wafers may be bonded together. Grooves may be formed either before or after bonding of the wafers. If the groove is formed by impact grinding, it must be formed before bonding. Gauges and their terminals may be defined in the gauge wafer by doping them to the requisite high concentration of boron before bonding the wafers, then etching away all of the undoped portion of the gauge wafer.

Alternatively, the whole bonded surface of the gauge wafer may be doped with boron so that the etching leaves a continuous sheet of gauge material from which gauges may be etched by a subsequent photolithographic step. This is similar to the bonded wafer approach described and claimed in UK Patent Specification 2093272 (corresponding to U.S, Application Serial No.

233,728, filed February 12, 1981), and the reader is now referred thereto.

For example, the gauge wafer will still be (110) 100 [ 1111, while the hinge wafer is (100) [ 1101 for easy and precise etching. This gives less difference in strain on the gauges and the associated hinge surface than does the square etch pattern into (110). Once the two wafers are bonded together, with the gauges positioned over their appropriate grooves or apertures which have been defined in the wafers, then the gauges are freed by etching away all of the gauge wafer except the gauges and their terminals. This approach is more complex in its execution, but offers dielectric isolation of the gauges, rather than diode isolation. Also, this allows the use of different crystal orientations in the gauge and substrate wafers. On the other hand, this 115 approach departs from one of the primary aspects of this invention, namely having the crystal structure of the gauge the same as its substrate support.

A piezoresistive transducer developed in accordance with the general procedures noted above is particularly appropriate for use in accelerometers, pressure transducers, and displacement gauges. The length of each typical individual gauge produced in accordance herewith, will be generally about 25 microns, while the width will be generally about 6 microns.

The preferred general steps or procedure of this invention in fabricating a piezoresistive transducer dice for use in an accelerometer 130 includes first selecting a silicon wafer. In this connection, it should be understood that a plurality of sensors are produced in a single wafer depending upon the form of sensor being developed in any particular application. Subsequently, the individual sensors are diced out of the wafer, once the sensors have been formed with their gauges, in accordance with this invention. After the wafer is selected, it is heavily oxidized. Subsequently, index marks are imposed on either side of the wafer photo 1 ithog raphica 1 ly in order to align the patterns on each side of the wafer. It should be pointed out here that with respect to each die formed on a wafer, gauges may be formed on one or both sides of the wafer, again depending upon the form of sensor being developed for a particular application.

Subsequent to imposing the index marks on each side by photolithographic means, apertures are opened in the oxide layer which are to be heavily doped to define the gauges and conductors therefore. After this is done, boron is implanted into the open areas on both sides in the amount of 1.5 x 10'6cm', sufficient to obtain boron in the amount of at least 5x 1019 atoms per cubic centimeter, and a depth within the range of between about 0.1 and 3 micrometers. The implantation should provide nearly equal doping on both sides. Subsequent to the implantation of the boron, the silicon wafer is annealed at a temperature of 9200C for about one hour. In this connection, for a more detailed discussion about general procedure of the kind carried out and discussed here, reference is made to the above noted U.K. Patent Specification No. 2093272. The same boron doping can be achieved by planar diffusion.

Once the annealing procedure has taken place, the etching patterns are opened on both sides photolithographically. Thus, the wafer is prepared for the etching procedure. Etching may be done by a potassium hydroxidewater-isopropyi alcohol bath.

Preferably, however, an ethylene diamine- pyrocatechol etch is utilized. In this connection, during this etching procedure, areas protected by oxide and areas heavily doped with boron do not etch. The etching procedure takes approximately four hours. Preferably, etching is to a depth of about 0.0022 inches assuming a wafer is 0.005 inches to leave a central hinge of 0.0006 inches. The depth should be sufficient to obtain a substantially level bottom surface of the groove below the gauges. Also, depth should be sufficient such that residual thickness at the bottom of the groove, considered as an elastic hinge, represents a small fraction of the bending stiffness in a system consisting of the formed hinge and its gauge.

After the etching procedure has taken place, all of the previously applied oxide is stripped and a thin oxide layer is grown on the wafer to protect the P-N junctions. Aluminium is then deposited on one or both sides to provide the metallic connections for the individual gauge or gauges. In 3 GB 2 128 404 A 3 this connection, once the aluminium has been deposited, then the patterns of the aluminium for forming the contact areas are photolithographically defined on the wafer.

Subsequently, the wafer is cut into the individual 70 dice with a diamond saw.

An embodiment of the present invention will now be described in more detail with reference to the accompanying drawings, in which:

Figure 1 is a view in perspective of a piezoresistive transducer illustrating the invention in which a single gauge is arranged on one side of its respective substrate; Figure 2 is a view in perspective of a further embodiment of piezoresistive transducer illustrating the invention in which two force gauges are arranged on one side of the substrate; Figures 3a-3j are diagrammatic views, in section, illustrating the sequential processing conditions of the invention, as the wafers are processed in accordance with this invention; Figure 4 is a view in perspective of a further embodiment of the invention in which the gauges are etched upon a cantilevered support; 25 Figure 5 is a view in perspective of a still further embodiment of the invention illustrating a form of invention in which the gauges are etched offset from the principal crystal direction; and Figure 6 is a view in section of a mesa supported gauge illustrating a further embodiment of the invention.

Referring to the drawings in which like reference characters refer to like parts throughout the several views thereof, Figure 1 illustrates a piezoresistive transducer 10, illustrating the invention, with a substrate 24 having a groove 25 defined therein undercutting the gauge 12. As can be seen in Figure 1, gauge 12 extends over groove 25 to pads 14, 16 at each end thereof.

There is a connection 18 between gauge pad 14 with the end 19 of substrate 24, while an opposite link or connection 20 maintains contact with gauge pad 16. There is a contact 22 to the substrate end 28. As will be appreciated, groove 25 defines a hinge 30 between the fixed end 28 of substrate 24, and the movable end 26 thereof.

A force is applied in the direction of arrow 32 on movable end 26, which causes movable end 26 to move around hinge 30 relative to fixed end 28, thus creating a strain in gauge 12, which is 115 measured electronically.

The sensitive elements or gauges formulated in the manner of the invention may be mounted in an electronic circuit for connection to a recording system whose design depends upon the ultimate application of the circuitry. For example, for use in a pressure transducer system, the gauges of the invention may be mounted in a Wheatstone bridge circuit in a pressure sensor similar to that shown in U.S. Patent 4,065,970.

Referring to Figure 2, piezoresistive transducer 34 is shown with dual gauges 36 formed on the top surface 60 thereof in accordance with the general procedures discussed above. The dual gauges 36 terminate atone end thereof in pad 58130 positioned on movable end 44 of substrate 42, while gauges 36 have individual pads 56 positioned on the fixed end 46 of substrate 42.

The pads 56 have electrical contact terminals 38 positioned thereon, while pad 58 has metallic area 40 formed to reduce electrical resistance of pad 58 between the adjacent ends of gauges 36.

The areas 38 and 40 may be comprised of aluminium.

As can be seen in Figure 2, hinge 52 is positioned midway between the top and bottom surface of substrate 42, as opposed to the arrangement shown in Figure 1. Thus, an upper groove 48 and a lower backside groove 50 are formed to define hinge 52. It is to be understood, from the showing in Figure 2, that a gauge pattern similar to that shown on top surface 60 of substrate 42 may be formed on the bottom surface thereof with the gauge pattern being nominally identical to that shown. The gauge patterns will be isolated from the substrate and from each other by the PN junctions. This arrangement is derived from the general processing conditions and steps noted above.

Referring now to Figures 3a-3j, a sequence of steps is shown for processing a single sided freed or suspended gauge piezoresistive transducer arrangement. Thus, as shown in Figure 3a, substrate 62 has formed thereon an oxidized layer 64 on the top and an oxidizing layer 66 on the bottom surface. Subsequent to the oxidizing step, indices are coordinated for processing both the top surface 64 and the bottom surface 66 of the substrate 62 by forming the coordinated indices 68, 70 therein. As can be seen in Figure 3c, top surface 64 is opened for doping at 72. Thereafter, boron from B20. is diffused into the open apertures to a concentration of 1020 boron/cc, which might give a sheet resistance of 6 ohms per square, for example. Figure 3d shows the diffused boron 74 in the open areas 72, as well as in the index pattern 68.

Following the boron diffusion step, both sides of substrate 62 are opened, as shown at 76, 78, respectively, (Figure 3e) with an etching pattern for the subsequent etching procedure. Subsequently, the etching procedure is carried out, preferbaly with an ethylene diaminepyrocatechol etch. The etch takes place to a depth of 0.0022 in 0.0050 inches (say 0.056 to 0. 130 mm) to undercut the gauges and leave a hinge of a thickness of about 0.0006 inches (say 0.0015 mm). As can be seen in Figure 3f, the etching forms grooves 48, 50 to define a hinge 52 at each point of etch. Also, as can be seen in Figure 3f, the coordinate index pattern arrangement 68, 70 is affected by the etch. In this connection, the original index marks are made immune to etching by boron doping. Index images may or may not open new index areas to the etch, as desired. The formed gauges 84, as can be seen in Figure 3f extend over grooves 48 in a manner similar to that shown in Figure 2.

Subsequently, the used oxide is stripped from the substrate 62 and a thin oxide coating is grown 4 GB 2 128 404 A 4 on both surfaces 64, 68 to form the arrangement as shown in Figure 3g. Following the growth of a thin oxide layer, a metal layer 80 is deposited on the top surface 64 of substrate 62, as shown in Figure 3h. The aluminium or metallized deposit 80 is then patterned to define the contacts or connecting links of the pads formed at each end of the gauges. Finally, the individual dice are cut from the wafer having been processed in accordance with the procedure discussed above, with the individual dice being in a form similar to that shown at 86 in Figure 3j.

As a further feature of this invention, particularly for low cost, high sensitivity pressure sensors, the relative ruggedness of a gauge on its own support extending across the groove has been found to be preferable to a fully freed or -floating- gauge. The strain energy needed for such a---mesa-supported gauge is about three times that needed for a freed gauge, but the resistance to handling damage is less expensive, as will be appreciated. Thus, if the etching is done into a (100) crystal surface, the walls of the etched cavities are 351 to 450 from vertical.

Conductive metallic films may be deposited and patterned up and down these slopes, which define a mesa supporting the gauges. When etching into (100) gauges are aligned [1101, as required for highest gauge factor, the gauges will not be undercut, as discussed above but will 95 persist on mesas to give gauges of relative ruggedness.

Figure 6 illustrates this form of invention in section in which gauge 150 is supported on its related mesa 150 above groove plane 154. In this 100 form, a neutral axis of bending 156 is formed near the hinge plane 154.

If, on the other hand, it is preferably to have the gauges etch-freed in this plane if they are -misaligned-, offsetting one end of a gauge by at 105 least the gauge width will allow the etching to undercut the gauge. Some additional angling of the gauge may be needed to allow the etch to smooth out the space defining the groove under the gauge. The angled gauge has a width of about 11 7.5 microns, a length of 37.5 microns, a depth of microns and a flat bottom width of about 15 microns.

The penalty for angling the gauge off the principal crystal direction [1101 is a reduction in 11 gauge factor. For example, a 131 angle to the gauge reduces gauge factor 19%. This is a relatively small penalty compared to the gain in sensitivity resulting in removal of the underlying material. Figure 5 is representative of a structure 120 of the kind discussed above in which gauges 126 are angled relative to the principal axis 145 of substrate 120. As can be seen in Figure 5, the applied force, as indicated by arrow 142, is against the movable end 122 of substrate 120 on hinge 140 around the fixed end 124 of substrate 120. Hinge 140 is defined by upper and lower grooves 136, 138 respectively. In this particular sensor element configuration, gauges 126 end in a single pad 128 at one end thereof while each individual gauge 126 terminates at the other end thereof in individual pads 130. Aluminium connections 132, 134 are deposited on these pads.

As a further feature of the invention, a cantilevered sensor may be utilized. Figure 4 shows an embodiment of sensor utilizing the cantilever. Thus, referring to Figure 4, sensor 94 is shown mounted on a base block 92. Sensor 94 may be mounted to base block 92 through the use of a clamp or the two parts may be bonded together using an adhesive. Sensor 94 has a fixed end 100 bonded to base block 92, while the movable end 102 is cantilevered from sensor 94.

Thus, movable end 102 reacts to forces in the direction of arrow 104 around hinge 114 defined by upper and lower grooves 116, 118 respectively. Gauges 98 are subjected to the strain during this movement in one direction and the electrical signal therein is picked up by contacts 106, 112 deposited on the pads 105, 107 respectively at each end of gauges 98. This particular form of sensor includes identical gauges deposited on the bottom surface of sensor 94 for sensing movement of end 102 in the reverse direction of force 104. Thus, leads 108 extend from contacts 106 while leads 110 extend from contacts deposted on the lower surface of sensor 94 and in contact with gauges mounted thereon. The assembly shown in Figure 4 may be employed as an accelerometer, for example, wherein the inertial force of the end 102 is the force measured by the system.

As will be appreciated from the above discussion the invention herein provides piezoresistive transducers utilizing sensor elements with gauges produced in situ on their substrates, which allows the use of stressed volumes of material smaller by a factor of hundreds from the stressed volumes previously thought practical. This increased sensor sensitivity can be applied to various types of transducers to produce very improved performance. Accelerometers utilizing the sensor 0 of the invention have an extremely high range, for example. A conventional accelerometer, for example, is calculated to have a resonance frequency of 161 KHz fora sensitivity of 1 micron volt per volt. By contrast, an etch-free gaugp arrangement in an accelerometer in accordance with this invention has a resonance frequency of 1.28 MHz for the same sensitivity. Furthermore, pressure transducers are developed of substantially smaller size with much greater sensitivity, high resonance frequency and good linearity, because of the small deflections required. They are substantially smaller than prior art force gauges, relatively simple in structure, easily manufactured, and therefore, less 125 expensive.

These sensing elements of the invention can be readily fabricated by mass production techniques, because they are formulated in situ, thus reducing the amount of handling necessary, particularly with respect to mounting the gauges therefor, on i -X GB 2 128 404 A 5 the supporting substrates. This makes the 65 methods and the products in accordance with this invention highly advantageous commercially, particularly with respect to the substantial decrease of required material needed for the stressed volume in the sensors.

While the methods and products herein disclosed form preferred embodiments of this invention, this invention is not limited to those specific methods and products, and changes can be made therein without departing 75 from the scope of this invention.

Claims (20)

Claims

1. A process for producing a strain sensitive element, the process comprising the steps of:

a) selecting a silicon crystal wafer; b) depositing in a first deposing step an oxide layer on both sides of the selected wafer; c) establishing coordinated indices on both sides of the wafer and the depositing step; d) on at least one side of the wafer in a first defining step in the oxide layer defining gages and the conductors therefor by opening apertures in the oxide deposit corresponding to the defined gauges and 90 conductors; e) in a second depositing step depositing boron into the aperture defined in the first defining step; f) subjecting the wafer to sufficiently elevated 95 temperatures for a sufficient period of time to anneal the wafer; g) opening an etching pattern on at least one side of the wafer to define grooves to be formed in the wafer over which the defined 100 gauges will extend; h) etching the etching pattern from the opening step by application of an etchant bath; i) stripping from the wafer the residual oxide; j) in a third depositing step depositing a thin 105 oxide layer on the wafer to protect the positive-negative junctions thereon; k) in a fourth depositing step depositing a metallic layer on at least one side of the wafer; 1) in a second defining step defining the conductors for the gauges in the metallic layer; and m) cutting the wafer into dice containing the defined gauges and conductors.

2. A process according to Claim 1, further characterized in that the first defining step, the opening step, and the fourth depositing are carried out on both sides of the wafer.

3. A process according to claim 2, further characterized in that the second depositing step is carried out in an amount sufficient to obtain diffused boron in the amount of at least 5x 1019 atoms per cubic centimeter, and a depth within the range of between 0.1 and 3 micrometers.

4. A process according to claim 1, further characterized in that the first defining step, the opening step, and the fourth depositing step are carried out on one side of the wafer.

5. A process according to Claim 4, further characterized in that the second depositing step is carried out in an amount sufficient to obtain diffused boron in the amount of 1010 boron per cubic centimeter.

6. A process according to any preceding claim, further characterized in that the subjecting step is carried out by ion implantation and holding the wafer at a temperature of 9200 for one hour.

7. A process according to any preceding claim, further characterized in that the etching step is carried out with a bath containing potassium hydroxide, water, and isopropyl alcohol.

8. A process according to any of claims 1 to 6, further characterized in that the etching step is carried out with a bath containing ethylene diamine, pyrocatechol and water.

9. A system for converting mechanical movement of two relative movable parts of a mechanical device into electrical signals, characterized by:

a) a substrate comprised of piezoresistive semi-conductive material; b) groove means extending across the substrate defining the relative movable parts; c) at least one strain sensitive element extending across the groove means and perpendicular thereto; d) the strain sensitive element being a unitary member having two end portions interconnected by and separated by an intermediate neck portion; e) the unitary strain sensitive element being derived from the same piezoresistive semiconductive material of the substrate; f) the end portions and neck portion of the strain sensitive element lying on a common axis; g) each of the end portions being joined to one of the relative movable parts of the substrate; and h) electrode means electrically connected to the end portions for detecting changes in electrical resistance between the end portions when the neck portion is subjected to stress resulting from relative movement of the substrate parts.

10. A system according to Claim 9, further characterized in that the strain sensitive element has a strain volume of 3x 1010 cubic centimeters of stressed material.

11. A system according to Claim 9 or 10, further characterized in that the groove means define a hinge between the relative movable p a rts.

12. A system according to Claim 11, further characterized in that the groove means is a single groove extending from one surface of the substrate to the hinge.

13. A system according to Claim 9, 10, or 11, further characterized in that a plurality of spaced apart strain sensitive elements extend across the groove; and the end portions of the plurality of strain sensitive elements are electrically 6 GB 2 128 404 A 6 interconnected by the electrode means.

14. A system according to Claim 11, further characterized in that the groove means is composed of two opposed spaced apart grooves extending toward each other from the top and bottom surface of the substrate to define the hinge.

15. A system according to Claim 14, further characterized in that a plurality of spaced apart strain sensitive elements extend across the grooves on the top and bottom surface of the substrata; and the end portions of the plurality of strain sensitive elements are electrically interconnected by the electrode means.

16. A system according to any of Claims 9 to 15, further characterised in that the axis of the strain sensitive element is parallel with the axis of the substrate.

17. A system according to any of Claims 9 to 40 15, further characterised in that the axis of the strain sensitive element is offset from the axis of the substrata; and one end of each of the strain sensitive elements is offset from the other end thereof along the substrate axis by an amount equal to the width of the neck portion thereof.

18. A system according to Claim 9, further characterized in that the unitary strain sensitive element is etched free from the substrata; and the end portions of the strain sensitive element are bonded to the substrata relative movable parts of the substrate.

19. A system according to Claim 9, further characterized in that mesa support means extend between the bottom surface of each strain sensitive element and the substrata; and the mesa support means are unitary with the end portions of the respective strain sensitive element and the related surface of the substrate upon which the mesa support means sits.

20. A transducer comprising a plurality of interconnected sensors each as defined in any of Claims 9 to 19.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1984. Published by the Patent Office. 25 Southampton Buildings, London, WC2A 1 AY, from which copies may be obtained.

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